Slender unreinforced masonry walls are likely to fail due to mechanism formation when they are subjected to eccentric compression loads causing second order bending effects. A practical analytical formulation is herein presented to provide an accurate calculation tool for these cases. The proposed method is compared with Eurocode-6, ACI-530, Southwell Plot method (semi empiric), a Finite Element Analysis and experimental tests. The results show that the proposed method and ACI-530 are accurate at calculating the load-bearing capacity of the most slender walls and the less slender cases respectively. The Southwell Plot method achieves the best performance although its applicability is limited.